Magnetorheological fluids (MRFs) are commercially available magnetic fluids which are currently used for a variety of applications. These include use in automotive parts: engine mounts, shock absorbers, and seat dampers [Phule, Pradeep P., and John M. Ginder, eds., xe2x80x9cThe Materials Science of Field-Responsive Fluidsxe2x80x9d MRS Bulletin, 19-21, August 1998; Ginder, John M., xe2x80x9cBehavior of Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 26-29, August 1998; Ginder, E. M. and Davis, C. S., xe2x80x9cShear Stresses in Magnetorheological Fluids: Role of Magnetic Saturation,xe2x80x9d Appl. Phys. Lett. 65 3410-3412, Dec. 26, 1994; Ashour, Osama, and Craig A. Rogers, xe2x80x9cMagnetorheological Fluids: Materials Characterization and Devices.xe2x80x9d J Int. Mat. Sys. Struct. 7: 123-130, March 1996]. Other applications cover a range from exercise equipment to aspherical optical lens polishing. In the area of vibration control and damping, earthquake resistant structures are built that utilize these fluids using semi-active control [Phule, Pradeep P., and Ginder, John M., eds., xe2x80x9cThe Materials Science of Field-Responsive Fluidsxe2x80x9d MRS Bulletin, 19-21, August 1998; Ginder, John M. xe2x80x9cBehavior of Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 26-29, August 1998; Ashour, Osama, and Craig A. Rogers. xe2x80x9cMagnetorheological Fluids: Materials Characterization and Devices.xe2x80x9d J. Int. Mat. Sys. Struct. 7 123-130, March 1996; Tang, X., X. J. Wang, W. H. Li, and P. Q. Zhang. xe2x80x9cTesting and Modeling of an MR Damper in the Squeeze Flow Modexe2x80x9d].
MRFs excel in these applications because their rheological properties are controlled over several orders of magnitude. Without an applied magnetic field, the typical MRF acts like a Newtonian fluid [Ginder, John M., xe2x80x9cBehavior of Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 26-29, August 1998; Dang, Anh, Liling Ooi, Janine Fales, and Pieter Stroeve, xe2x80x9cStress Measurements of Magnetorheological Fluids in Tubes.xe2x80x9d Ind. Eng. Chem. Res. 39:2269-2274, 2000]. When a field is applied, a dipole moment is induced in the particles in the MRF. This causes the particles to align xe2x80x9chead-to-tailxe2x80x9d and form chains of particles. Thus, these particles form structures parallel to the magnetic field [Ginder, John M., xe2x80x9cBehavior of Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 26-29, August 1998]. The MRF becomes a weak viscoelastic solid when the chain or column structures form. As a result, the rheological properties of the materials change. As the magnetic field increases, the material exhibits a rapid and nearly reversible increase in yield stress. Because of the change in material properties under the influence of a magnetic field, the MRF properties are controlled and therefore provide a new means of controlling electromechanical devices. [Phule, Pradeep P., and John M. Ginder, eds. xe2x80x9cThe Materials Science of Field-Responsive Fluidsxe2x80x9d MRS Bulletin, 19-21, August 1998; Jolly, Mark R., Jonathan W. Bender, and J. David Carlson xe2x80x9cProperties and Applications of Commercial Magnetorheological Fluidsxe2x80x9d SPIE 5th Int. Symposium on Smart Structures and Materials San Diego, Calif., Mar. 15, 1998.]
While MRFs may be similar to ferrofluids, they also have important differences. They are composed of three components like ferrofluids; thus, they have a carrier fluid, magnetic particles, and additives [Raj, K. B. Moskowitz, and R. Casciari xe2x80x9cAdvances in Ferrofluid Technologyxe2x80x9d J. Magn. Magn. Mat. 149 174-180, 1995]. However, the particles used in ferrofluids are superparamagnetic iron oxide nanoparticles (xcx9c5-10 nm). [Phule, Pradeep P., and John M. Ginder, eds., xe2x80x9cThe Materials Science of Field-Responsive Fluidsxe2x80x9d MRS Bulletin, 19-21, August 1998; Raj, K. B. Moskowitz, and R. Casciari xe2x80x9cAdvances in Ferrofluid Technologyxe2x80x9d J. Magn. Magn. Mat. 149 174-180, 1995]. As a result, they do not exhibit a shear yield stress like MRFs while under an applied magnetic field. [Phule, Pradeep P., and John M. Ginder, eds. xe2x80x9cThe Materials Science of Field-Responsive Fluidsxe2x80x9d MRS Bulletin, 19-21, August 1998; Ashour, Osama, and Craig A. Rogers, xe2x80x9cMagnetorheological Fluids: Materials Characterization and Devices.xe2x80x9d J. Int. Mat. Sys. Struct. 7 123-130, March 1996.] This is due to a reduced tendency to form chains under a magnetic field. Thus, while viscosity changes can be observed, they are small. [Ashour, Osama, and Craig A. Rogers, xe2x80x9cMagnetorheological Fluids: Materials Characterization and Devices.xe2x80x9d J. Int. Mat. Sys. Struct. 7 123-130, March 1996; Odenbach, Stefan, Thomas Rylewicz, and Michael Heyen. xe2x80x9cA Rheometer Dedicated for the Investigation of Viscoelastic Effects in Commercial Magnetic Fields.xe2x80x9d J. Magn. Magn. Mat. 201 155-158 1999.] The applications, as a result, are much different. In addition to being used in seals, the ferrofluids have applications in stepper motors and sensors. [Raj, K. B. Moskowitz, and R. Casciari xe2x80x9cAdvances in Ferrofluid Technologyxe2x80x9d J. Magn. Magn. Mat. 149 174-180, 1995.]
For an MRF, magnetic particles, such as iron, can be suspended in a fluid. Under a magnetic field, these particles form chains [Phule, Pradeep P., and John M. Ginder, eds., xe2x80x9cThe Materials Science of Field-Responsive Fluidsxe2x80x9d MRS Bulletin, 19-21, August 1998; Phule, Pradeep P., xe2x80x9cSynthesis of Novel Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 23-25, August 1998; Huang, Jiun-Yan and Pik-Yin Lai, xe2x80x9cFormation and Polarization of Dipolar Chainsxe2x80x9d Physica A 281 105-111, 2000] that significantly increase the yield stress of the material. The carrier fluid acts as the medium for other components. Suspended in the medium are the magnetic particles that form chains when a magnetic field is applied. Finally, additives are used to provide stability to the mixture, corrosion control, lubrication, anti-oxidants, pH shifters, dyes and pigments, salts, and deacidifiers. [Phule, Pradeep P. and John M. Ginder, eds. xe2x80x9cThe Materials Science of Field-Responsive Fluidsxe2x80x9d MRS Bulletin, 19-21, August 1998; Dang, Anh, Liling Ooi, Janine Fales, and Pieter Stroeve. xe2x80x9cStress Measurements of Magnetorheological Fluids in Tubes.xe2x80x9d Ind. Eng. Chem. Res. 39 2269-2274, 2000; Phule, Pradeep P. xe2x80x9cSynthesis of Novel Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 23-25, August 1998; A. Fuchs, F. Gordaninejad, C D. Blattman, and G. Hamann. xe2x80x9cMagneto-rheological Polymeric Gel Materials.xe2x80x9d Provisional U.S. Patent, February 2000.]
Typically, the carrier medium is a silicone oil or hydrocarbon fluid. [Phule, Pradeep P., and John M. Ginder, eds. xe2x80x9cThe Materials Science of Field-Responsive Fluidsxe2x80x9d MRS Bulletin, 19-21, August 1998; Dang, Anh, Liling Ooi, Janine Fales, and Pieter Stroeve. xe2x80x9cStress Measurements of Magnetorheological Fluids in Tubes.xe2x80x9d Ind. Eng. Chem. Res. 39 2269-2274, 2000.] This is because it exhibits many of the properties that are desirable in MRF. Ideally, the fluid should be thermally stable, have a high boiling point, be nonreactive (especially with the dispersed material) and be nontoxic. Also, the fluid should contribute to the stability of the mixture, but at the same time enable the redispersibility of the magnetic particles. The temperature dependence of the medium""s viscosity is also very important, and is in fact the dominating factor in the operating range of the MRF. For the stability of the MRF, the carrier fluid should be noncorrosive and nonreactive with the magnetic particles and other ingredients. Finally, the fluid should not cause sealing problems in the device in which it will be used. [Ginder, John M., xe2x80x9cBehavior of Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 26-29, August 1998; Phule, Pradeep P. xe2x80x9cSynthesis of Novel Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 23-25, August 1998.]
The dispersed phase of an MRF usually is a soft magnetic material like iron particles of 1-10 um size [Phule, Pradeep P. and John M. Ginder, eds., xe2x80x9cThe Materials Science of Field-Responsive Fluidsxe2x80x9d MRS Bulletin, 19-21; August 1998.] Several important factors must be considered in the choice of the dispersed phase. First, the volume fraction of the magnetic materials in the fluid is chosen. For the iron system, usually 0.3 to 0.5 volume fraction of carbonyl iron is used in the fluid. This leads to a reasonable yield stress but does not have the higher off-state viscosity of higher volume fractions. Several problems occur when the particles are too small. They are more influenced by the carrier fluid than the larger particles. They are also more sensitive to temperature. Also, the possibility of agglomeration increases. Nano-MR fluids are described in the literature [Phule, Pradeep P. xe2x80x9cSynthesis of Novel Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 23-25, August 1998; Luan, H. Martin, Claudius Kormann, and Norbert Willenbacher, xe2x80x9cRheology on Magnetorheological (MR) Fluids.xe2x80x9d Reol. Acta., 35 417-432, 1996]. BASF researchers created stable (by using polyelectrolyte adsorption) nano-MR fluids using ferrites ( less than 100 nm). However, the yield stress is only xcx9c6 kPa and it is temperature sensitive [Phule, Pradeep P. xe2x80x9cSynthesis of Novel Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 23-25, August 1998].
The manufacture of iron and iron-based alloys is achieved using several methods: decomposition of iron pentacarbonyl, sol-gel ultrasonic decomposition of organometallic precursors, plasma torch synthesis, electroexplosion of metal wires, chemical reduction and precipitation, and laser ablation. Preferably, soft magnetic materials like iron are used for their high saturation magnetization. Fe-Co alloys have the highest saturation magnetization (xcx9c2.4 T), but cost and unavailability make them undesirable unless the higher material strength is needed. Ferromagnetic materials such as manganese-zinc ferrite and nickel-zinc ferrite (xcx9c2 xcexcm in size) have a lower saturation magnetization and thus they have a lower maximum yield stress [Phule, Pradeep P. xe2x80x9cSynthesis of Novel Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 23-25, August 1998].
A wide variety of MR materials have been developed [Ginder, J. M., (1996), xe2x80x9cRheology Controlled By Magnetic Fields,xe2x80x9d Encyclopedia of Applied Physics, Vol. 16, pp. 487-503; Ginder, J. M., Sproston, J. L., (1996), xe2x80x9cThe Performance of Field-Controllable Fluids and Devices,xe2x80x9d Proceedings of Actuator 96, 5th International Conference on New Actuators, pp. 26-28; Ginder, J. M., Davis, L. C., Elie, L. D., (1996), xe2x80x9cRheology of Magnetorheological Fluids: Models and Measurements,xe2x80x9d International Journal of Modern Physics B, Vol. 10, Nos. 23and24, pp. 3293-3303; Ginder, J. M., Davis, L. C., (1994), xe2x80x9cShear Stresses in Magnetorheological Fluids: Role of Magnetic Saturation,xe2x80x9d Appl. Phys. Lett., Vol. 65, No. 26, pp. 3410-3412; Shiga, T., Okada, A., Kurauchi, T., (1993), xe2x80x9cElectroviscoelastic Effect of Polymer Blends Consisting of Silicone Elastomer and Semiconducting Polymer Particles,xe2x80x9d Macromolecules, Vol. 26, p. 6958-6963]. These include materials with differing particulate material, particle size, host material, volume fraction, and additives. These materials include ferrofluids, MR fluids, magnetic powders, and MR elastomers [Ginder, J. M., Nichols, M. E., Elie, L. D., Tardiff, J. L., (1999), xe2x80x9cMagnetorheological Elastomers: Properties and Applications, Smart Materials Technologies,xe2x80x9d Ed. by M. Wuttig, Proc. of SPIE Vol. 3675, in press; Kelso, S. P. and Gordaninej ad, F., (1999), xe2x80x9cMagneto-Rheological Fluid Shock Absorbers for Off-Highway, High-Payload Vehicles,xe2x80x9d Proceedings of the 1999 SPIE Conference on Smart Materials and Structures, Long Beach, Calif.].
Several approaches for development of MRFs are documented in the patent literature. U.S. Pat. No. 5,985,168 describes the use of a bridging polymer to modify the surface of the iron particles. This approach leads to improved stability and redispersibility. In this patent only three thermoset polymers are described: polyvinylpyrollidone, polyethyleneamine and poly(4-vinlypyridine). The polymeric material does not appear to be crosslinked.
Organic polymers are also used to coat the surface of iron particles, as described in U.S. Pat. No. 5,989,447. This patent describes many families of polymers which are used and exhibit reduced abrasiveness and produce high stability with regard to settling. The use of polyelectrolytes to coat magnetic particles is described in U.S. Pat. No. 5,508,880. Iron coated with monolayers, bilayers and multiple layers are taught in: K. Nozawa et al., xe2x80x9cChemical Modification of Alanethiol Monolayers for Protecting Iron against Corrosion,xe2x80x9d (1997); G. Kataby et al., xe2x80x9cSelf-assembled monolayer coatings of iron nanoparticles with thiol derivatives,xe2x80x9d (1996); M. Wolpers et al., Surface analytical investigations of metal surfaces modified by langmuir-Blodgett films of silanes,xe2x80x9d (1990); M. Wolpers et al., xe2x80x9cSEM and SAM imaging of silane LB films on metallic substrates,xe2x80x9d (1990); G. Kataby et al., xe2x80x9cThe adsorption of monolayer coatings on iron nanoparticles: Mossbauer spectroscopy and XANES results,xe2x80x9d (1998); S. Ramachandran et al., xe2x80x9cSelf-assembled monolayer mechanism for corrosion inhibition of iron by imidazolines,xe2x80x9d (1996); G. Kataby et al., xe2x80x9cCoating carboxylic acids on amorphous iron nanoparticles,xe2x80x9d (1998); G. Kataby et al., xe2x80x9cCoating of amorphous iron nanoparticles by long-chain alcohols,xe2x80x9d (1997); T. Prozorov et al., xe2x80x9cEffect of surfactant concentration on the size of coated ferromagnetic nanoparticles,xe2x80x9d (1998); W. Gao et al., xe2x80x9cSelf-assembled monolayers of alkylphosphonic acids on metal oxides,xe2x80x9d in C. Grozinger and L. Reven (1996); Y. Liu et al., xe2x80x9cLayer-by-layer electrostatic self-assembly of nanoscale Fe3O4 particles and polyimide precursor on silicon and silica surfaces,xe2x80x9d (1997); S. Nilsson et al., xe2x80x9cNovel organized structures in mixtures of a hydrophobically modified polymer and two oppositely charged surfactants,xe2x80x9d (2000); H. Yu et al., xe2x80x9cMolecular orientation and electrochemical stability of azobenzene self-assembled monolayers on gold: an in situ FTIR study,xe2x80x9d (2000); N. E. Schlotter et al., xe2x80x9cFormation and structure of a spontaneously adsorbed monolayer of arachidic on silver,xe2x80x9d (1986); and H. Shiho et al., xe2x80x9cMagnetic compounds as coatings on polymer particles and magnetic properties of the composite particles,xe2x80x9d (1999).
The use of polymeric thixotropes is described in U.S. Pat. Nos. 5,645,752; 5,683,615; 5,382,373; 5,705,085; and WO 94/10693, which disclose the use of polymeric materials as thixotropes with which magnetic particles are mixed to form magnetorheological materials. These publications do not appear to disclose the use of a continuous covalently crosslinked polymeric gel (as opposed to hydrogen-bonded gels) or non-stoichiometric ratios of polymer components to effect partial crosslinking as a means for controlling viscosity.
Magnetorheological (MR) dampers are semi-active devices that contain magneto- rheological fluids. Activation of the damper""s built-in magnetic field causes a fast and dramatic change in the apparent viscosity of MR fluid contained in the damper. The fluid changes state from liquid to semi-solid in milliseconds. The result is an infinitely variable, controllable damper capable of large damping forces. MR dampers offer an attractive solution to energy absorption in mechanical systems and structures. This is because they can be battery operated, require minimal power for operation, and have a broad range of capabilities; for example, the absence of mechanical valving (for flow control) in the damper, high and low temperature tolerances, insensitivity to impurities penetration, fluid stability, and long operational life. Most importantly, they are inexpensive devices to manufacture, utilize and maintain. A controllable damper is described in U.S. Pat. No. 6,019,201.
All publications referred to herein are incorporated by reference to the extent not inconsistent herewith.
Magnetorheological materials are typically comprised of magnetizable particles suspended in a carrier material. A magnetorheological material exhibits rapid and reversible changes that are controllable by an applied magnetic field. The shear stress and viscosity of such a material is related to whether the material is in the presence of a magnetic field, termed the on-state, or the absence of a magnetic field, termed the off-state. In the on-state, the magnetic particles align with the magnetic field and increase the shear yield stress and viscosity of the material over its off-state value.
Stable polymeric magnetorheological (MR) gels have been developed with higher off-state viscosities than silicone oils, resulting in higher coefficients of damping and better fatigue resistance. MR gels with off-state viscosities between about 20 cp and about 200 cp lower than currently commercial available fluids are also provided herein.
The magnetorheological material provided herein comprises magnetic particles and a carrier material which is a polymeric gel, preferably a partially-crosslinked polymeric gel. Preferred MR gels of this invention are made by a method comprising forming said polymeric gel in the presence of said magnetic particles. Partial crosslinking is achieved by controlling reaction conditions such as time, temperature, catalysts, etc. as known to the art, and in two-component polymer systems is controlled by reacting the components in non-stoichiometric amounts. The term xe2x80x9cpartially crosslinkedxe2x80x9d means the gel contains a measurable amount of crosslinking but measurably less than all crosslinking possible. In this invention the crosslinking is covalently bonded. For partially crosslinked polymeric gels of this invention, nonstoichiometry of the components leads to the desired degree of crosslinking.
Both thermosetting and thermoplastic polymers are useful in this invention.
Magnetorheological materials are also provided having a selected off-state viscosity, and comprising magnetic particles and a carrier material which is a polymeric gel. There may or may not be a diluent or non-gel carrier fluid present. Viscosity of the fluid may be controlled by degree of crosslinking of the polymer, amount of plasticizer (also referred to herein as a diluent), and amount and type of magnetic particles. As is known to the art, plasticizers compatible with the polymer system being used should be selected.
The magnetic particles can be any magnetic particles known to the art. The particle component of the magnetorheological material of the invention can consist essentially of any solid which is known to exhibit magnetorheological activity, e.g., made of compounds which exhibit paramagnetic, superparamagnetic or ferromagnetic activity. Such particles may be made of iron, iron oxide, iron nitride, iron carbide, carbonyl iron, chromium dioxide, low carbon steel, silicon steel, nickel, cobalt, and mixtures thereof. Iron oxide includes all known pure iron oxides, such as ferric and ferrous oxides, e.g., ferrites and magnetites. The magnetic particles can be comprised of alloys of iron, such as those containing aluminum, silicon, cobalt, nickel, vanadium, molybdenum, chromium, tungsten, manganese and/or copper. Typically, the magnetic particles are in the form of metal powders prepared by processes well known to those skilled in the art. Typical methods for the preparation of metal powders include the reduction of metal oxides, grinding or attrition, electrolytic deposition, metal carbonyl decomposition, rapid solidification, or smelt processing. Various metal powders that are commercially available include iron powders, reduced iron powders, insulated reduced iron powders, and cobalt powders. Preferred particles of the present invention are iron powders, reduced iron powders, iron oxide powder/iron powder mixtures and iron oxide powder/reduced iron powder mixtures. Most preferred are reduced carbonyl iron particles. Magnetic particles with high saturation magnetization, such as iron/cobalt alloys are preferable for this application. These iron alloys are selected to provide high yield stress.
The particle size has a great influence on the rheology of the on and off states of the fluid. For larger particles (5-7 xcexcm) the yield stress is greater than for smaller particles (xcx9c2 xcexcm). Particles larger than 10 um have increased settling and thus form less stable MRF. Magnetic particles may be present at between about 10 to about 95% by weight of the material. The amount of magnetic particles should be sufficient to provide the required apparent on-state shear yield stress and viscosity, preferably 5 to 50 volume percent, more preferably 15 to 40 volume percent, based on the total volume of the magnetorheological material. The magnetic particle component preferably has an average particle size ranging from about 5 nm to about 10 xcexcm or about 100 nm up to about 10 xcexcm, or about 1 xcexcm to about 10 xcexcm. Preferably, the average particle diameter of the particles is at least about 0.03 micrometers, more preferably at least about 0.05 micrometers. Preferably, the magnetic particles are present at a mass fraction of around 80% (or about 50 percent by volume) when high yield stresses are desired.
The remainder of the material, e.g., about 20 to about 99 mass percent, comprises a carrier component. A volume fraction of about 50 to about 95 volume percent, preferably about 60 to about 85 volume percent based on the total volume of the magnetorheological material, is also useful. The carrier component preferably comprises or consists essentially of a covalently crosslinked polymeric gel component capable of providing the desired shear yield stress and viscosity, preferably a thermosetting or thermoplastic polymer, polyurethane, modified polyurethane (including those using reactions with isocyanate, isocyanurate, urea, allophanate, biuret, oxazolidone, carbodiimide or cyclic imide), and silicone, epoxy, acrylic, polyamide, polycarbonate, polyester, polyanhydride, and polyimide polymers. The polymeric may be made using a polymer as described in U.S. Pat. No. 5,645,752, incorporated herein by reference.
Additionally, non-polymeric materials may be added to the material to adjust viscosity, preferably a natural fatty oil, mineral oil, polyphenylether, dibasic acid ester, neopentylpolyol ester, phosphate ester, polyester, cycloparaffin oil, paraffin oil, unsaturated hydrocarbon oil, synthetic hydrocarbon oil, perfluorinated polyether or halogenated hydrocarbon. Other optional additives include thixotropic agents, rust inhibitors, carboxylate soaps, antioxidants, lubricants, and viscosity modifiers, all known to the art.
The magnetorheological material is preferably in the form of a continuous gel phase, in which all the polymer is covalently crosslinked to form a network rather than being made up of gel-coated particles in a liquid carrier. Magnetorheological materials of this invention may be made by forming the polymeric gel in the presence of the magnetic particles to provide improved dispersion stability, especially when the gel coats the particles in the form of monolayers (e.g., self-assembled monolayers [SAMs]), bilayers, multiple layers, or thin films. A diluent may be added to the magnetic particles and the polymeric gel precursor prior to polymerization. Magnetorheological materials of this invention may also be formed by adding the magnetic particles after the polymerization reaction has occurred. Or, the polymerization reaction may be conducted, diluent added, and then the magnetic particles added. In any event, the magnetorheological material is preferably in the form of a continuous gel phase, rather than being made up of gel-coated particles in a liquid carrier. In one embodiment, the polymeric gel is substantially uniformly distributed throughout said material.
The reaction conditions for making the polymer gels, especially the ratios of the reactants, are adjusted to vary the amount of cross-linking and gel formation. For a given polymer, the greater the amount of cross-linking and gel formation the greater the viscosity and the less the magnetic particle settling.
For a given polymeric gel system, the reaction chemistry typically involves one or more monomer reactant(s) and a plasticizer. Sample reaction chemistries for different polymer systems are given below.
For a polyurethane system appropriate reactants include polyether-polyols or polyester-polyols and aromatic or cycloaliphatic isocyanate. An appropriate plasticizer for the reactants is an aromatic-ester. Other plasticizers known to the art may also be used.
For a silicone system, appropriate reactants include vinyl terminated silicone polymers and silane groups. An appropriate plasticizer for these reactants is a silicone oil. Other plasticizers known to the art may also be used.
For an epoxy system, appropriate reactants include diglycidal ether of bis-phenol A (DGEBA) or Novalak resins and aromatic, cycloaliphatic or aliphatic amines or Lewis base catalysts. An appropriate plasticizer for this system is diglycidal ether of butane diol (DGEB). Other plasticizers known to the art may also be used.
The desired viscosity of the magnetorheological material in the off-state to be selected depends upon the proposed application. For some applications, minimizing the off-state viscosity is important. For applications requiring high damping forces, the ability to increase the off-state viscosity can be extremely valuable. The selected off-state viscosity can be between about 20 and about 5,000,000 cp.
The magnetorheological materials of this invention have very low settling rates of particles and preferably possess ideal initial (off-state) viscosities for selected applications, such as land-based applications, aerospace applications, and earthquake control. For dampers, low to medium viscosities are required, e.g. about 20 to about 10,000 cp. For clutches, low viscosities, e.g. about 20 to about 200 cp viscosities are required.
MRF additives are necessary to prevent agglomeration and settling. As the particles settle and the distance between them decreases, the small level of remnant magnetization could play a role in agglomeration. Some of the materials used as additives are nanostructured silica, fibrous carbon, and various polymers. Nanoscale silica forms a coating on magnetic particles as a thixotropic network. [Phule, Pradeep P. xe2x80x9cSynthesis of Novel Magnetorheological Fluidsxe2x80x9d MRS Bulletin, 23-25, August 1998.]
The present invention provides a method for making a magnetorheological material having a selected off-state shear yield stress and viscosity comprising mixing a polymeric gel carrier material with a selected quantity of magnetic particles or polymerizing the carrier in the presence of magnetic materials. The carrier material and quantity of magnetic particles are selected together so that the resulting magnetorheological material has the desired off-state shear yield stress and viscosity. The desired off-state shear yield stress and viscosity is selected by determining the required off-state output characteristics of a magnetorheological device that employs the magnetorheological material to effect a damping force, torque or resulting pressures. The magnetorheological material composition having the desired viscosity may be selected by preparing such materials in accordance with the teachings herein to provide selected viscosities, testing the viscosities of known magnetorheological material compositions, or by other means known to the art. For example, polymer selection, degree of polymerization, reactant stoichiometry (e.g., polyols and isocyanates for polyurethane systems, vinyl-terminated silicone polymers and silane groups for silicone systems, and ethers, resins, amines and catalysts for epoxy systems) as well as addition of reactive or non-reactive plasticizers, percent and type of particulate material, cure time and temperature, as known to the art, can be varied to achieve the desired off-state viscosities.
By selection of the appropriate polymer system with the desired off-state viscosity, material strength and fatigue life is improved.
The present invention also provides a method of controlling the output characteristics of a magnetorheological device containing a magnetorheological material of the present invention, comprising selecting a magnetorheological material of the desired off-state viscosity and dependence of apparent viscosity on magnetic field and controlling the magnetic field to change the apparent viscosity of said material. The desired off-state viscosity and range of on-state apparent viscosities are selected by determining the required off-state and on-state output characteristics of the magnetorheological device employing the magnetorheological material. The off-state shear yield stress and viscosity of the magnetorheological material and the dependence of the apparent viscosity on the magnetic field can both be selected by selecting the appropriate magnetorheological material composition as described above. The material behaves in a Newtonian manner in the off-state. For dampers and clutches the on-state yield stresses should be as high as possible. As is known to the art, shear stress equals yield stress plus shear rate times plastic viscosity.
Such methods for controlling output characteristics of a magnetorheological device having a selected magnetic field in its on-state, containing a magnetorheological material comprise: selecting a desired off-state viscosity for said magnetorheological fluid; selecting a desired on-state apparent viscosity for said magnetorheological fluid; and providing in said device a magnetorheological material having said selected off-state viscosity and said on-state apparent viscosity in said magnetic field.
The present invention provides a method for making a magnetorheological material comprising selecting a desired off-state viscosity for said material; selecting a desired off-state shear yield stress; selecting a quantity of magnetic particles; providing an amount of a polymeric gel carrier or precursor thereof calculated to produce said desired off-state viscosity when formulated with said quantity of magnetic particles; and formulating said material by combining said carrier or precursor with said magnetic particles. The term polymeric gel carrier precursor refers to monomers used to form a polymeric gel prior to crosslinking.
Magnetorheological fluids comprising magnetic particles coated with monolayers, self-assembling monolayers, bilayers or multiple layers of polymeric gel are also provided herein. Self-assembled monolayers are desirable for good particle dispersion and because they take up minimum possible volume, higher mass content of magnetic particles is achievable. They are typically prepared from alkanethiols as is known to the art. Bilayers may also be prepared by means known to the art using surfactant bilayers and provide stable dispersions. Multilayers or thin films can be made using fluidized bed coatings. The carrier phase may be liquid, a continuous gel phase, or a continuous crosslinked polymeric gel phase.